The long-term goal of this proposal is to understand, in detail, the molecular mechanisms of telomere homeostasis through comprehensive characterization of the telomere protein-DNA complex. Telomeres ensure genome integrity by facilitating chromosome end replication through telomerase. In addition, telomeres also protect the chromosome ends from DNA repair and degradation activities. Mutations in telomerase subunits or telomere components have been linked to premature aging and cancer. Maintaining proper telomere structure and length is required for accomplishing both its chromosome end replication and end protection tasks. The structure-function relationship of this complex is a central question in understanding the mechanisms of telomere homeostasis. We will exploit the genetically amendable S. pombe model system and employ a combination of biochemical, structural and genetic tools to achieve the following specific aims: 1. Map the respective protein-protein interaction interfaces between Tpz1 and its interaction partners and examine the dynamics of the Tpz1-centered complex formation in vitro. 2. Reveal the functional roles of Tpz1 mediated interactions in telomere length homeostasis and chromosome end protection. 3. Achieve a comprehensive structural analysis of the Tpz1-centered complex using single particle cryo-electron microscopy (cryo-EM) and X-ray crystallography. Accomplishment of the proposed aims will provide new and significant mechanistic insights into the structure-function relationship and the dynamics of the telomere complex, and may also provide the foundation for development of new therapeutic approaches against diseases caused by telomere dysfunction.